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Paul B. MacCready Jr.

An inexpensive cloud theodolite has been developed to help an observer find cloud positions and movements. For velocity and direction measurements the observer notes the time and direction the cloud moves across a pattern which is optically superimposed on the sky. A chart is provided which simplifies the task of finding cloud speeds (if height is known), directions, and positions from the measurements.

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Paul B. MacCready Jr

Abstract

Satisfactory measuring techniques have been developed which overcome the four, principal experimental problems arising in the investigation of atmospheric turbulence.

1. The data are made reproducible for statistical analysis by recording them on a portable magnetic tape recorder.

2. and 3. The problems of measuring wind velocity fluctuations over broad ranges (frequencies between 0 and 50 cycles per second, and magnitudes between 0.25 and 20 meters per second) are solved by using hot-wire anemometers together with non-linear amplifiers.

4. Measurements at elevations up to several hundred meters are made feasible by the use of a multi-cable tethering system for a balloon which supports the measuring devices.

The techniques permit the measurement of velocity distributions, frequency spectra, autocorrelation coefficients, and heat flux at any elevation.

The difficulty of constructing such measuring and analysis equipment is balanced by the relative simplicity with which a complete statistical analysis of the data may be performed.

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Paul B. MacCready Jr.

Abstract

Studies by various investigators have shown that rising spherical balloons exhibit self-induced lateral motions which can introduce errors when the balloons are used for wind soundings. Balloons operating in the subcritical Reynolds number regime tend to move in a regular, spiral or zigzag path, and then even if the lateral motions are appreciable, the balloon path can rather accurately represent the horizontal wind averaged vertically over a thickness of 15 or more baboon diameters. Typical expandable neoprene balloons smaller than 1-m diameter at launch are within the subcritical regime at all altitudes and so would be expected to be good for soundings all the way from the ground up. Experimental data are presented for several sizes of 100 gm neoprene balloons, and the data are in agreement with this expectation.

Reynolds number effects, rise rates, and peak altitudes are considered for several balloon types. Neoprene balloons feature high peak altitudes and relatively fast ascent at the higher altitudes. A roughened 2-m diameter superpressure *Jimsphere” yields accurate soundings from ground to its peak of about 20 km, and initially rises fairly rapidly. Smooth superpressure balloons give accurate detailed soundings over a limited altitude range (about 12 to 20 km for 2-m diameter, for example).

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Paul B. MacCready Jr.

This paper constitutes a brief review of the main concepts relating to dynamic response characteristics. It gives the quantities used to define the characteristics (a response time or distance for devices like a simple temperature probe or rotating cup anemometer; both a delay factor and a damping ratio for a direction vane). It shows how to obtain these quantities experimentally by noting the sensor response to a step change of the variable being measured. Graphs are presented, showing the response of the sensors to sinusoidal inputs, by which measured statistical data can be corrected for the sensor characteristics.

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Paul B. MacCready Jr

Abstract

Experimental data on the energy spectra, correlation coefficients and energy isotropy of atmospheric turbulence at low elevations are presented. It is shown that the very simple formulas derived with the aid of Kolmogoroff's Similarity Hypothesis are remarkably consistent with the experimental evidence of this and other research projects. The Similarity Hypothesis, although only directly applicable to turbulent eddies small enough to be isotropic, appears in some instances to be useful in its simplest form for atmospheric problems with scales varying between one cm and a thousand km. Some possible applications are described together with the regions where the Similarity Hypothesis might be expected to hold.

Several measurements of vertical turbulent heat flux at various elevations were made with rapid-response equipment. A systematic variation of the heat flux with elevation was noted even at the low elevations involved.

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Paul B. MacCready Jr.

Abstract

A universal turbulence standardization technique is described which is based quantitatively on the atmospheric turbulence itself rather than on the effects it products on an aircraft. It provides a single turbulence intensity number which may be measured continuously in flight in a variety of ways, and, with knowledge of the aircraft type and speed, can be linearly related to the rms value of vertical accelerations of the aircraft. The universal intensity number concept as applied here is an extension for aircraft use of the inertial subrange concept of atmospheric turbulence. In the inertial subrange all the statistical properties of turbulence of wavelengths less than a few hundred meters (the wavelength range which dominates gust loading for most aircraft) can be simply related to a single number. The intensity measurement also has value as a basic meteorological parameter, and has proven to be particularly useful in diffusion studies.

The intensity scale is derived from various turbulence power spectrum studies. A larger background of data on turbulent distributions is also available because existing statistical discrete gust and continuous gust data can be fitted approximately to this standardization procedure, and meteorological studies have also given estimates of the distribution of this intensity number. The intensity number can be found from virtually any device moving through the air, as well as by some indirect meteorological means. A particular version of a universal turbulence indicator is described.

The universal turbulence intensity rating provides a standard for communication about turbulence and for the collection of statistical turbulence information. It also provides a quantitative method of relating aircraft response and pilot “feel” to the turbulence, so that the pilot can select or preselect the appropriate speed for specific missions.

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Paul B. MacCready Jr.

Abstract

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Paul B. MacCready Jr.

Cloud seeding operations took place over a 10,000 square mile area in Central Arizona during the first four months of the winter of 1951. The natural precipitation for these months in this area was estimated by a simple correlation method utilizing an adjacent unseeded region. The actual precipitation consistently exceeded the estimated natural precipitation. The apparent precipitation increases were roughly proportional to the amount of cloud seeding equipment used. Although the correlation technique applied is rather inexact, it seems sufficiently accurate to make the consistent precipitation increases suggestive of a large positive effect from cloud seeding.

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Paul B. MacCready Jr.
and
Clement J. Todd

Abstract

A continuous particle sampler has been developed which captures atmospheric particles in a Formvar solvent liquid film. The Formvar solution completely encapsulates the particle; then as the solvent evaporates the film hardens quickly, preserving a replica of the particle. The method yields exact replicas of ice crystal shapes. Liquid droplets are somewhat flattened by surface tension during replication and so a calibration factor is necessary to ascertain the original droplet size from the replica. This calibration factor has been found experimentally by using spores distributed in the droplets to show the volume of the droplet creating each replica. The sampler provides continuous information on particle concentration, particle sizes, and whether the particles are ice or water. The Formvar solution is ordinarily applied to transparent 16-mm movie leader film, and the replicas are viewed by projecting the film with a stop-motion microscope projector.

Several versions of the instrument have been built and used on cloud physics research aircraft. Special adaptations have been made for ground use: to sample fogs, to measure crystals in a freezing nuclei counter, and to collect snowflakes.

Various design compromises are required in the operational instruments to overcome, over a broad range of meteorological conditions, problem such as those associated with film coating, droplet encapsulation, droplet migration and spurious crystal growth. These factors and others associated with particle collection, data reduction and interpretation are discussed. Examples of the field use of the sampler are given.

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Paul B. MacCready Jr.
and
Donald M. Takeuchi

Abstract

Samplings of the unmixed cores and the mixed edges of convective clouds at various heights were made in an aircraft using standard instrumentation and a Continuous Particle Collector (CPC). The unmixed cores are defined as regions where constant values are indicated on the hot wire type, liquid water content meter. The various correction factors relating these CPC replicas to the true droplet and drop situation in the cloud are not well established, but the observations are nevertheless sufficiently accurate 1) to give some useful data on the relative size distributions of small droplets, and 2) to show that drizzle-size drops can develop quickly within the cloud core. The evolution of the distributions of small droplets (<40μ diameter) with height is shown. Droplet concentrations are of the order of 1000 cm−3 near cloud base, and some correlation between concentration and upcurrent strength at cloud base is noted. The large droplets from which drizzle-size drops evolve can be present, even at cloud base, in concentrations of about 20 liter−1. Possible sources of these large droplets are discussed. Graupel was encountered at −5 and −6 C, mostly outside the convective cores, presumably resulting from the freezing and riming of the larger liquid drops at relatively warm temperatures.

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